Momentum Flux in Turbulent Boundary Layers with Weakly Unstable Stratification

• Published in: Journal of the Physical Society of Japan 2022-05, Vol.91 (5)
• Main Authors: Mizuno, Yoshinori, Yagi, Toshimasa, Mori, Kazuyasu
• Format: Article
• Language: English
• Subjects: Boundary layer thickness; Clustering; Momentum transfer; Stratification; Turbulent boundary layer; Wind tunnel testing; Wind tunnels
• ISSN: 0031-9015; 1347-4073
• DOI: 10.7566/JPSJ.91.054402

We examine the momentum flux in boundary layers with weakly unstable stratification through wind-tunnel experiments. If the intensity of stratification is evaluated by −δ/L (> 0 for unstable cases), where δ is the boundary layer thickness and L is the Obukhov length, the cospectrum of the streamwise and wall-normal velocity fluctuations is found to be enhanced at large scales comparable to δ with the increase in −δ/L. On the other hand, the meanlength of time durations where the momentum transfer is active is much shorter than the wavelengths at which the cospectrum is enhanced by stratification. We then introduce a method to detect the clustering of the active time durations. It is found that the mean length of clusters is little sensitive, but the interval between clusters becomes longer for stronger stratification. The sum of mean length and interval of clusters is comparable to the flux-enhanced scales found in the cospectrum. The appearance of long quiescent durations in stratified boundary layers leads to the increase in the cospectrum at large scales.